1. Field of the Invention
The present invention relates generally to wireless local loop and cellular communication systems. More particularly, the present invention relates to a wireless communication system which dynamically adjusts the power of signals transmitted over reference channels from a base station to minimize power spillover to adjacent communication cells.
2. Description of the Related Art
Wireless communication systems have rapidly become a viable alternative to wired systems due to their inherent advantages. Wireless systems enable subscribers to freely move throughout the operating range of a service provider and even into the territory of other service providers while using the same communication hardware. Wireless communication systems are also utilized for applications where wired systems are impractical, and have become an economically viable alternative to replacing aging telephone lines and outdated telephone equipment.
One of the drawbacks with wireless communication systems is the limited amount of available RF bandwidth. There is a constant desire to improve the efficiency of these systems in order to increase system capacity and meet the rising consumer demand. A factor that degrades the overall capacity of wireless communication systems is signal power spillover between adjacent cells or base stations. This occurs when the power of signals transmitted by a base station in a particular cell exceeds the boundary of that cell, otherwise known as the operating range. The spillover becomes interference to adjacent cells and degrades the efficiency of the system. Accordingly, minimizing spillover is one of the most important issues in wireless communications system design.
Forward power control (FPC) is used to minimize spillover by adjusting the power level of signals transmitted from the base station to subscriber units on assigned channels. The FPC operates in a closed loop wherein each subscriber unit continuously measures its received signal-to-noise ratio and transmits an indication back to the base station of whether the base station should increase or decrease the transmit power to that subscriber unit. The closed loop algorithm assists in maintaining the transmit power level from the base station at a minimum acceptable level, thereby minimizing spillover to adjacent cells.
FPC, however, cannot adjust the power level for reference channels such as the pilot signal, broadcast channel or paging channel. Since there is no closed loop algorithm that operates on these channels, the reference channel transmit power level for the worst case scenario is typically used. The power level is generally more than what is required for most subscriber units, resulting in spillover to adjacent cells.
There have been prior attempts to overcome the problem of spillover. U.S. Pat. No. 5,267,262 (Wheatley, III) discloses a power control system for use with a CDMA cellular mobile telephone system including a network of base stations, each of which communicates with a plurality of subscriber units. Each base station transmits a pilot signal which is used by the mobile units to estimate the propagation loss of the pilot signals. The combined power of all base station transmitted signals as received at a mobile unit is also measured. This power level sum is used by the mobile units to reduce transmitter power to the minimum power required. Each base station measures the strength of a signal received from a mobile unit and compares this signal strength level to a desired signal strength level for that particular mobile unit. A power adjustment command is generated and sent to the mobile unit which adjusts its power accordingly. The transmit power of the base station may also be increased or decreased depending upon the average noise conditions of the cell. For example, a base station may be positioned in an unusually noisy location and may be permitted to use a higher than normal transmit power level. However, this is not performed dynamically, nor is the power correction based upon the total transmit power of the base station.
Accordingly, there exists a need for an effective method for controlling the power level of reference channels transmitted from a base station.